Reciprocating piston engines are extensively used because of their excellent combustion gas sealing and lubricating properties. However, the reciprocating engine tends to have a complex structure, be large in size, has high production cost, and cause vibrations. It is difficult to realize complete combustion in a reciprocating engine because the available combustion strokes depend on a crank angle not greater than 180 degrees. Furthermore, the crank mechanism properties set an upper limit on the conversion efficiency from combustion gas pressure to output power (torque, horsepower). The crank radius is determined according to the cylinder capacity. It is difficult to increase the crank radius and, accordingly, the output performance. In addition, in the case of a four-cycle engine, every two rotations of the crank shaft create one combustion stroke, hampering downsizing of the engine. In order to deal with this, the engine rotation speed is increased for higher output horsepower. This is disadvantageous because combustion performance is reduced as the engine rotation speed is increased.
Over the past 130 years or so, various rotary engines (rotary piston type internal combustion engines) have been proposed. However, they are all imperfect except for the Wankel rotary engine. Rotary engines are divided into two major groups including: a unidirectional rotary engine in which the rotor does not have an eccentric motion and the Wankel rotary engine in which the rotor has eccentric motion.
Approximately 12 years ago, the inventor of the present application proposed an unidirectional rotary piston type rotary engine cited in Patent Document 1, which has an annular operation chamber outside the outer periphery of the rotor. The rotor comprises a pressuring/pressured member partitioning the annular operation chamber. The housing comprises first and second oscillating partitions that partition the annular operation chamber, wherein the first partition opens/closes an auxiliary combustion chamber. Two sets of spring assemblies for elastically biasing the first and second partitions are respectively provided.
With this rotary engine, the annular operation chamber formed outside the outer periphery of the rotor and the two sets of spring assemblies make the engine greater in size. The first and second partitions and rotor make line-contact, not area-contact, with problems relating to hermetic sealing and lubricating properties.
Conversely, Patent Documents 2 to 5 have proposed various unidirectional rotary piston type rotary engines. The rotary engine described in Patent Document 2 has an approximately 240 degrees arc-shaped intake/compression groove formed on a sidewall of the rotor, a partition biased by a spring and partitioning the intake/compression groove, an arc-shaped expansion/exhaust groove formed on the outer periphery of the rotor, and a compression/explosion chamber formed in a protrusion of the housing.
The rotary engine of Patent Document 3 is a vane type rotary engine having a rotor eccentrically installed in the circular retention hole of a housing, an output shaft passing through the center of the rotor, eight vanes mounted on the rotor in a radially reciprocating manner, and an auxiliary combustion chamber formed on the outer periphery side of the circular retention hole.
The rotary engine of Patent Document 4 has a rotor concentrically installed in the circular retention hole of a housing, an intake groove formed by cutting out the outer periphery of the rotor into an arc (a crescent) shape, a partition mounted on the housing and abutting the outer periphery of the rotor, and a cam mechanism for radially moving the partition.
The rotary engine of Patent Document 5 has a housing, a nearly oval rotor retained in a circular retention chamber in the housing, two partitions biased by springs, a timing rotor retained in a circular hole situated next to the circular retention chamber via an middle side plate, an arc-shaped main combustion chamber formed on the outer periphery of the timing rotor, an auxiliary combustion chamber formed outside the outer periphery of the main combustion chamber, a heating plug facing the auxiliary combustion chamber, and a secondary injection nozzle. Fuel-air mixture pressurized by the rotor in the intake/compression chamber is introduced into the auxiliary combustion chamber, where it is compressed and ignited. The combustion gas is introduced into the expansion/exhaust chamber among the circular retention chambers via the main combustion chamber, enabling the combustion gas to work on the rotor.    Patent Document 1: WO96/11334;    Patent Document 2: Japanese Patent Laid-Open Publication No. S52-32406;    Patent Document 3: U.S. Publication No. 5,979,395;    Patent Document 4: Japanese Laid-Open Patent Publication No. H10-61402; and    Patent Document 5: Japanese Laid-Open Patent Publication No. 2002-227655